Probes which simultaneously measure temperature and pressure of a gas stream are well known in the art and are particularly useful for gas turbine engine applications. One such probe is shown in commonly owned U.S. Pat. No. 3,343,417 to C. A. Peek, Jr. That probe includes a longitudinally extending body having a teardrop cross-sectional shape. Longitudinally spaced apart pressure taps or passages in the leading edge of the probe body intersect a longitudinally extending pressure measuring cavity. A separate longitudinally extending temperature measuring cavity is disposed within the probe body downstream of the pressure cavity. Longitudinally spaced apart inlets to the temperature cavity are located downstream of the pressure cavity near the trailing edge of the probe. Scoops adjacent the temperature cavity inlets direct the gas flow into the temperature cavity. A single outlet to the temperature cavity located at one end thereof has a thermocouple element disposed therein which reads the temperature of the gas at the cavity outlet.
To obtain an accurate and timely reading of the temperature of a gas at a particular location within a gas path it is desirable to have that gas flow over the thermocouple junction at near or greater than the same velocity at which it was flowing within the gas path; and the junction should be as close as possible to the point within the gas path at which the temperature is to be measured. Otherwise, the temperature which is being measured may not accurately reflect the temperature of the gas at the gas path location of interest; and it may not be the temperature at the time of interest. In the Peek, Jr. patent described above there is necessarily a time delay for the gases to reach the sole thermocouple junction; also the gases may change temperature as they travel from the probe inlet to the thermocouple junction. Pressure gradients on the external surface of the probe body just upstream of the temperature cavity inlets may also affect the flow velocities entering the temperature cavity. Stagnant or low velocity air within the cavity and which entered the probe at one moment of time may become mixed with gases entering the probe at a different moment of time, such that the temperature reading may be less accurate and less indicative of the temperature in the free gas stream at the moment of temperature reading.
Commonly owned U.S. Pat. No. 4,605,315 describes a temperature probe which is a part of a gas turbine engine strut. In that patent a scarfed tube at the strut leading edge provides a fixed stagnation point at the throat of the probe inlet. The inlet and outlet dimensions are predicated on having the velocity of the gas flow over the thermocouple junction within the temperature cavity bear a relationship to the free-stream velocity of the gas. Note, also, the prior art shown in FIG. 1 of U.S. Pat. No. 4,605,315. In that figure a small tube having an inlet at the leading edge 12 of the probe directs fluid over a thermocouple 14. The tube passes directly through an unnumbered cavity which is only shown in cross section in FIG. 1. That cavity is a longitudinally extending pressure cavity analogous to the pressure cavity 33 of U.S. Pat. No. 3,343,417. Pressure taps (not shown) at longitudinal locations different from the locations at which temperature readings are taken intersect that cavity. The tubes passing through the pressure cavity are undesirable since they adversely affect the pressure readings within the pressure cavity.
Another patent which shows a pressure probe within the leading edge of a strut is commonly owned U.S. Pat. No. 4,433,584 to Kokoszka et al. Other patents showing the state of the art of instrumentation probes of either the pressure or temperature sensing type are U.S. Pat. Nos. 2,414,370; 2,571,422; 2,971,997; 3,000,213; 3,075,387; 3,348,414; 3,451,862; 3,497,398; 3,605,495 (combination pressure and temperature probe); and 4,244,222.